1. Field of the Invention
The present invention generally relates to an ISDN (Integrated Services Digital Network) electronic exchange having a central processing unit and a storage unit, and more particularly to an extension agent system installed in such an ISDN exchange.
2. Description of the Prior Art
In an extension agent system of an electronic exchange, an extension agent number is assigned to a plurality of terminals, such as telephone sets. When a call addressed to the extension agent number is received, the terminals are searched for in a predetermined sequence in order to determine whether or not an idle terminal is available. Then, the received call is transferred to an idle terminal which is first accessed.
The basic interface of the ISDN uses a plurality of channels (2B and D) for an interface. In an extension agent system configured in conformity with the ISDN basic interface, even if one of the channels of an interface is busy, this interface is recognized to be idle when the other channel is idle. In a case where a speech terminal is connected to one of the B channel in each interface forming the extension agent system and a terminal other than a speech terminal is connected to the other channel therein, each interface is recognized to be idle even when the speech terminal connected to each interface is busy. Hence, the extension agent system cannot operate.
FIG. 1 is a block diagram of an electronic exchange having a conventional extension agent system, and FIG. 2 is a flowchart showing the operation of the conventional extension agent system. The electronic exchange shown in FIG. 1 comprises a switching network (NW) 10, a central processing unit (CC: hereinafter simply referred to as a controller) 11, and a main memory (MM) 12. Terminals A-D connected to the switch 10 belong to an extension agent group. The main memory 12 includes an extension agent table 13, which has idle/busy information 13a and hunting group information 13b. The idle/busy information 13a show whether the terminals A-D belonging to the identical agent group are respectively busy or idle. The hunting group information 13b show a sequence of hunting in order to obtain an idle terminal.
Referring to FIG. 2, the controller 11 confirms, in step S1, completion of receipt of an extension agent number assigned to the agent group having the terminals A-D. For example, the extension agent number is aft identification number of the terminal A which has priority over the terminals B-D with respect to the hunting procedure. Further, in step S1, the controller 11 translates the received number and recognizes that the received number is the extension agent number. In step S2, the controller 11 refers to the busy/idle information 13a and the handling group information 13b stored in the table 13, and determines whether the terminal A located at the top of the handling sequence is idle or busy. When the terminal A is idle, the controller 11 makes a connection between a terminal sending the call and the terminal A in step S4. When it is determined, in step S2, that the terminal A is busy, the controller 11 refers to the table 13, and selects the terminal B which has the second priority in step S3. Then, the controller 11 determines whether or not the terminal B is idle or busy in step S2. In this manner, all the terminals indicated by the hunting group information 13b are sequentially accessed until an idle terminal is found. If it is determined that all the terminals A-D are busy, the controller 11 sends a busy tone to the calling terminal.
If the above-mentioned extension agent system is applied to an ISDN electronic exchange (see CCITT Recommendation Q. 931, "ISDN USER-Network Interface Layer 3 Specification For Basic Call Control", for example), the following problem will occur. As has been described previously, one interface between an user and the network 10 includes a plurality of channels (two B channels and one D channel). Each of the subscribers A-D has two B channels respectively having a bit rate of 64 Kbps and one D channel having a bit rate of 16 Kbps. It will now be assumed that each of the subscribers A-D respectively has only a speech-system terminal (normal telephone set). In this case, the controller 11 refers to the hunting group information 13b in response to receipt of an extension agent number, and determines whether or not the subscriber A is idle or busy. If the subscriber A is communicating with another subscriber via one of the two B channels, the other B channel is idle. In this case, the controller 11 tries to make a connection between the calling terminal and the idle B channel because all the B channels are not busy. A terminal other than the speech-system terminal is not connected to the extension of the subscriber A. Hence, the controller 11 receives no response from the subscriber A, and does not make a connection with the calling terminal. That is, the controller 11 does not hunt the subscriber B.